New Relic exporter updates.

[a-feld]

Description:

This PR includes the following updates to the New Relic collector:

Improvements

• Requests are now retry-able via configuration option (defaults to retries enabled). Permanent errors are not retried
• The exporter monitoring metrics now include an untagged summary metric for ease of use
• Improved error logging to include URLs that fail to post messages to New Relic

Bugfix

• Configuration unmarshalling did not allow timeout value to be set to 0 in the endpoint specific section
• Request cancellation was not propagated via context into the http request
• The queued retry logger is set to a zap.Nop logger as intended

CC @alanwest

[alanwest]

@tigrannajaryan This PR is ready when you have a chance. Thanks!